And very, very often, just plain bad. Sorry.
But book trailers are a new concept, and the quality of a nascent art form is always iffy. With book trailers, authors of extremely varying skills and background have to either cobble together their own movie or spend unknown thousands hiring someone to make it for them, of further unknown quality.
And who watches book trailers anyway?
Well, judging from my research checking Youtube View stats for hundreds of trailers...